1. Field of the Invention
This invention relates generally to Vertical Centrifugal Separators and more particularly to improvements made to such a Centrifugal Separator to improve its performance in drying oil and gas well cuttings in an offshore environment.
2. General Background
Oily drill cuttings often cannot be discharged directly into the environment due to their adverse effect upon the environment and, therefore, must be processed for disposal in costly disposal wells. Additionally, because of the great value of the residual oil and chemicals contained in them, it has been a common practice to treat the oil drill cuttings in order to produce a solid material that can be disposed into the environment surrounding the well site or returned into the well from which it came without injury to the environment or interference with the well. One method of treating these oily drill cuttings has been through the use of a chemical washing system. In this system, the oily drill cuttings are treated with various chemicals, including detergents, with relatively intense mixing. Then, this mixture is resolved into relatively oil-free solids (i.e., the drill cuttings) and a recovered liquid phase which is a mixture of water, oil, and the detergents which were employed in the chemical wash system. Burial or re-injection then disposes of the solids. However, these solids may still contain sufficient oil and/or chemicals that, upon contact with bodies of water, such as surface waters, lakes or the ocean, produce unacceptable levels of toxins detrimental to preserving the environment in the best possible form. In addition, the liquid phase must be treated to separate the oil from the bulk water phase so that the water portion can be discharged or otherwise disposed without pollution problems. The separated oil and expensive drilling fluids are usually recovered and utilized for various uses such as fuel or be returned into the blending of additional oil based drilling muds and the like.
Examples of the chemical wash system are described in U.S. Pat. Nos. 2,266,586, 3,860,019, and 3,766,997. Various other systems have been proposed for removing excess residue and chemicals from drill cuttings and transforming them into a solid material that can be returned to the surface environment or injected into the earth formation. For example, various thermal systems for flashing off the oily residue from the drill cutting solids through the use of thermal incineration have been proposed. An example of such incineration is described in U.S. Pat. Nos. 3,693,951, 2,266,586 and 4,139,462. The cuttings are heated to elevated temperatures above 500 degrees Fahrenheit for extended periods of time. Then, these heated cuttings are moved through a chamber until all the volatile materials are vaporized to leave an oil-free solids residue which could be disposed safely in the environment. This thermal procedure is relatively expensive in that it requires large amounts of an inert gas to prevent internal explosions by the contact with air of the heated solids at excessive temperatures. The relatively large amounts of the inert gas complicate the recovery of liquid materials from the chamber because of the undesired high levels of gas flow into condensers and the like. The greatest disadvantage in this particular procedure for treating oily drill cuttings is the danger of explosions in the system should air or other oxidizing gas enter into contact with the heated oil vapors produced by the excessive heating of the oily drill cuttings. Should the flow of inert gas be terminated through accident or inadvertence, air entering in contact with these oily vapors could produce a very serious explosion and fire. Such an arrangement is not acceptable in the area surrounding an oil well, especially while it is being drilled.
The above described prior art procedures for treating oily drill cuttings have other serious disadvantages, especially when they are to be employed on offshore drilling platforms. For example, large amounts of chemicals must be transported at great expense to offshore facilities. In addition, these offshore platforms do not have any surplus of steam, gas, electrical, or other energy sources. Thus, a procedure for treating the oily drill cuttings must be self-sufficient relative to the operations on the offshore platforms. In addition, the treatment procedure for the oily drill cuttings must be safe to operate, not require extensive retention time, operate without interference or hindrance to the drilling operations conducted on the offshore platform, while yet producing solids from the drill cuttings which can be disposed of safely and without any injury to the environment at the drilling site. In addition, the system for the treatment of oily drill cuttings at the drilling site, and especially on an offshore platform, must not require a constant supply of chemicals, fuel, nitrogen or other materials for its operation.
In addition, drying systems must meet stringent regulations that may effect the use of such equipment on an offshore platform where space is at a premium. Regulations dictate that equipment provided for use on offshore facilities after design and construction of the facility must meet certain height and weight restrictions in accordance with the equipment location. Such restrictions serve to induce equipment manufacturers to produce equipment with the lowest possible footprint with emphasis on efficiency.
It has been found that drill cuttings need not necessarily be incinerated to remove and recover residual cutting oils and expensive chemicals. Such oils and chemicals are routinely being removed and recovered by compression and separation as demonstrated by U.S. Pat. Nos. 6,279,471 and 6,170,580. However, it is still advantageous for the cuttings to be as dry as possible for transport and further processing for injection into the earth formation.
As discussed above, the drying operation for drill cuttings is a secondary operation of the separator system in which the primary consideration is to remove the liquids and render the residual oils and chemicals harmless to the environment through incineration. In cases where further processing of the drill cuttings prior to discharge or transport is still required, complete removal of the residual residue is unnecessary and drying the cuttings to improve handling, transport and further treatment becomes the primary objective. It is therefore essential that the most efficient method for drying drill cuttings be found.
Centrifugal separators are widely used as a very efficient method for separating fluids from solids. However, they are not generally considered to be driers and are not generally configured with any form of heat ducts due to the relatively low retention time of the materials passing through the separator.
In general, vertical, centrifugal separators such as is described in U.S. Pat. No. 5,256,289 include a housing containing a drive mechanism to which is connected both a flight assembly and a screen assembly. The separator further includes an inlet for induction of the material to be separated. Induced material is captured by the flight screen assembly, separation occurring as the material migrates downwardly with liquids or very small particles present on or in the material being forced outwardly through a fine screen into a space between the screen and the housing by centrifugal force. The majority of the liquids are then drawn off and the solids are generally ejected from an outlet assembly located below the rotor drive assembly. The outlet assembly usually is defined as a conical discharge bin for depositing the solids in a container or further conveyed to other locations for disposition, thereby making the dryer quite high.
The present dryer system utilizes a centrifugal separator as a dryer and is arranged to satisfy all of the requirements for use in connection with its adaptation to oil well drilling and especially in connection with its placement upon an offshore platform without the disadvantages of the prior art and, in addition, providing for self-sufficient operation with minimum operator attention and absolute fail-safe operation.
The present invention utilizes a vertical, centrifugal separator for drying drill cuttings prior to transport or further processing. The separator is adapted to receive scavenged heat from any source and is further adapted to include internal conveyers, thereby lowering the overall operating profile and increasing cuttings retention time within a heated environment.